Communications devices are increasingly using pluggable modules that plug into a host via a connector, which may be an optical connector. Some components of the communications device will be on the pluggable module, with the other components being on the host, and communication between the components on the host and the components on the module will take place over the connector. Optics for the system are located on the pluggable module (as well as some electronic control circuitry), and the remainder of the device components (e.g. the electronic signal processing) may be located on the host.
Until recently, signalling over the optical connection was digital. However, as bandwidths increase in optical communications, complex modulation formats and Nyquist pulse shaping mean that the data transmitted can be treated as essentially analogue rather than digital. The electronics at the end points (i.e. at the host) are predominantly digital. Therefore, digital-analogue convertors (DACs) are used to translate between the two domains. The DAC is generally located in the host.
The DAC receives a digital electrical input signal and outputs an analogue electrical signal. The output analogue signal is then used as an input to an optical modulator (which has a further optical input for a laser carrier wave), which outputs the modulated optical signal for transmission on an external optical connection. The modulator is generally located in the pluggable module, which may have multiple independent modulators which are used to build up a final signal.
The analogue nature of the signalling can cause problems which are not faced by digital methods at a lower bitrate. Due to the characteristics of the transmission path, there will be differing levels of attenuation for different frequencies of the transmitted signal, i.e. different frequency components of the signal will experience different changes to magnitude and/or phase, which will cause distortion in the overall signal. The relationship between the change in magnitude of a transmitted sine wave and the frequency of the wave is called the magnitude response of the transmission path. The distortion experienced by an arbitrary transmitted signal can be calculated if the magnitude and phase response is known over the frequency range of the signal (e.g. the range of components determined by Fourier analysis). The inverse of this distortion can then be calculated, i.e. the waveform that must be transmitted to ensure that the desired waveform arrives at the receiver. This process is called pre-emphasis, since certain frequency components are emphasised in order to counteract the magnitude response.
There will naturally be some variance between devices, connectors, and the connection quality each time a connector is plugged in, which will cause distortion between the DAC and the modulator in the transmit circuit, and between the demodulator and the digital signal processor in the receiver circuit. This variable signal distortion can be partially compensated for in the receiver circuit following demodulation, provided the signal-to-noise ratio is high enough. However, distortion on the transmit circuit can make the signal much harder to distinguish from the noise introduced on the optical connection, which results in a significant loss of signal quality.